A variety of drug delivery systems, including automatic drug delivery systems, are known. Because the consequences of delivering an inappropriate dosage (e.g., too much or too little) of a drug can be life threatening, it is of critical importance that drug delivery systems be extremely accurate. Drug delivery systems that are configured to deliver medication to patients must be configured to prevent even unlikely accidental delivery events. In particular, drug delivery systems that electrically delivery drug to a patient, including transdermal or other electrotransport drug delivery devices, be prevented from accidentally providing drug to the patient.
The term “electrotransport” as used herein refers generally to the delivery of an agent (e.g., a drug) through a membrane, such as skin, mucous membrane, or nails. The delivery is induced or aided by application of an electrical potential. For example, a beneficial therapeutic agent may be introduced into the systemic circulation of a human body by electrotransport delivery through the skin. A widely used electrotransport process, electromigration (also called iontophoresis), involves the electrically induced transport of charged ions. Another type of electrotransport, electro-osmosis, involves the flow of a liquid. The liquid contains the agent to be delivered, under the influence of an electric field. Still another type of electrotransport process, electroporation, involves the formation of transiently-existing pores in a biological membrane by the application of an electric field. An agent can be delivered through the pores either passively (i.e., without electrical assistance) or actively (i.e., under the influence of an electric potential). However, in any given electrotransport process, more than one of these processes may be occurring simultaneously to a certain extent. Accordingly, the term “electrotransport”, as used herein, should be given its broadest possible interpretation so that it includes the electrically induced or enhanced transport of at least one agent, which may be charged, uncharged, or a mixture thereof, regardless of the specific mechanism or mechanisms by which the agent actually is transported.
In general, electrotransport devices use at least two electrodes that are in electrical contact with some portion of the skin, nails, mucous membrane, or other surface of the body. One electrode, commonly called the “donor” or “active” electrode, is the electrode from which the agent is delivered into the body. The other electrode, typically termed the “counter” or “return” electrode, serves to close the electrical circuit through the body. For example, if the agent to be delivered is positively charged, i.e., a cation, then the anode is the active or donor electrode, while the cathode serves to complete the circuit. Alternatively, if an agent is negatively charged, i.e., an anion, the cathode is the donor electrode. Additionally, both the anode and cathode may be considered donor electrodes if both anionic and cationic agent ions, or if uncharged dissolved agents, are to be delivered.
Furthermore, electrotransport delivery systems generally require at least one reservoir or source of the agent to be delivered to the body. Examples of such donor reservoirs include a pouch or cavity, a porous sponge or pad, and a hydrophilic polymer or a gel matrix. Such donor reservoirs are electrically connected to, and positioned between, the anode or cathode and the body surface, to provide a fixed or renewable source of one or more agents or drugs. Electrotransport devices also have an electrical power source such as one or more batteries. Typically, one pole of the power source is electrically connected to the donor electrode, while the opposite pole is electrically connected to the counter electrode. In addition, some electrotransport devices have an electrical controller that controls the current applied through the electrodes, thereby regulating the rate of agent delivery. Furthermore, passive flux control membranes, adhesives for maintaining device contact with a body surface, insulating members, and impermeable backing members are some other potential components of an electrotransport device.
Small, self-contained electrotransport drug delivery devices adapted to be worn on the skin for extended periods of time have been proposed. See, e.g., U.S. Pat. No. 6,171,294, U.S. Pat. No. 6,881,208, U.S. Pat. No. 5,843,014, U.S. Pat. No. 6,181,963, U.S. Pat. No. 7,027,859, U.S. Pat. No. 6,975,902, and U.S. Pat. No. 6,216,033. These electrotransport agent delivery devices typically utilize an electrical circuit to electrically connect the power source (e.g., a battery) and the electrodes. The electrical components in such miniaturized iontophoretic drug delivery devices are also preferably miniaturized, and may be in the form of either integrated circuits (i.e., microchips) or small printed circuits. Electronic components, such as batteries, resistors, pulse generators, capacitors, etc. are electrically connected to form an electronic circuit that controls the amplitude, polarity, timing waveform shape, etc. of the electric current supplied by the power source. Other examples of small, self-contained electrotransport delivery devices are disclosed in U.S. Pat. No. 5,224,927; U.S. Pat. No. 5,203,768; U.S. Pat. No. 5,224,928; and U.S. Pat. No. 5,246,418.
Constant current supplies for variable resistance loads such as those appropriate for use with an electrotransport drug delivery device have been previously described, however such systems require the use of the cathode to determine the voltage and/or current at the cathode. For example, U.S. Pat. No. 5,804,957 to Coln describes a constant current supply system for a variable resistance load. This system includes a constant current circuit connected to a second output terminal (e.g., anode) for providing a predetermined current to the load (patient) and a constant control circuit. A voltage supply control circuit monitors the voltage at the second terminal across the constant current circuit and adjusts the voltage supply to maintain the second terminal at a preselected voltage for maintaining the predetermined current to the variable resistance load. See, e.g., FIG. 1, which illustrates a prior art system including a voltage control circuit that directly monitors the cathode (via a comparator “throttle” element 140).
However in some variations it may be beneficial to control and monitor the applied current without directly monitoring the second patient terminal (e.g., cathode). This configuration allows separation of the control aspect of the circuit from the risk management aspect of the circuitry.
For example, described herein are methods, devices and systems for monitoring and controlling electrotransport drug delivery devices including indirectly monitoring and controlling the circuit not directly connected to the patient terminal (e.g., cathode) using a switching element.